Suppression of B lymphopoiesis at a lymphoid progenitor stage in adult rabbits

Abstract

B lymphopoiesis in rabbits is robust early in ontogeny, but is arrested by 16 weeks of age at which time no proB or preB cells are found in bone marrow (BM). To determine if BM cells from adults retain B lymphopoietic potential, we transferred BM from adult green fluorescent protein (gfp) transgenic rabbits into young rabbits. We found gfp+ preB cells arising in the young recipients, indicating that BM cells from adults can differentiate into B cell precursors. We identified a population of MHCII-IL-7-binding BM cells in adults that collectively expresses Tdt, EBF and Pax5-genes known to be expressed in murine lymphoid progenitors. Upon co-culture with OP9 or OP9 delta-like 1+ stromal cells, we found that these cells both expanded in number and differentiated into B and T cell precursors, respectively, showing that early lymphoid progenitors, designated rLP for rabbit lymphoid progenitors, are present within the MHCII-IL-7-binding BM cell population. Further, IL-7 was required for rLPs to expand and differentiate into B cell precursors in vitro. The arrest of B lymphopoiesis in adults, however, is not likely due to the absence of IL-7, because the level of IL-7 transcripts was higher in BM from adults than in young rabbits. B lymphopoiesis was re-initiated in adults after sub-lethal irradiation as shown by the reappearance of B cell precursors and the presence of B cell receptor excision circles in BM. We conclude that B lymphopoiesis in adults is suppressed at a lymphoid progenitor stage (MHCII-IL-7 binding) of development.

In vitro requirement for periostin in B lymphopoiesis

“…2292 (data not shown). As shown previously, the cells that grew out from rabbit BM were confirmed as B lineage on the basis of their expression of CD79a ( Figure 5C in Kalis et al 7 ). We repeated these experiments with the use of murine BM cells and found an 18-fold reduction in the number of cells arising from Postn siRNA OP9 cells compared with cells arising from the GFP siRNA OP9 cells ( Figure 2C).…”

“…6 Lymphoid progenitors are present in the BM of adult rabbits as evidenced by the finding that total BM cells from adults differentiated into B-cell precursors after adoptive transfer into young recipients and also that rabbit lymphoid progenitors differentiated into B-and T-cell precursors when cocultured with OP9 or OP9-DL1 stromal cells. 7 These data suggested to us that the suppression of B lymphopoiesis in the BM of adult rabbits might be mediated through age-related changes in the BM environment. To investigate this hypothesis, a cDNA representational difference analysis (cDNA-RDA) was performed to identify genes downregulated in BM stromal cells from adults compared with BM stromal cells from young rabbits.…”

“…6 The proliferation and differentiation of HSCs to B-lineage cells requires vital signals from the BM microenviroment, including cytokines, growth factors, and adhesion molecules secreted by stromal cells and the ECM. [38][39][40] Previous investigation of the mechanism responsible for the observed arrest of B lymphopoiesis in rabbits led Kalis et al 7 to suggest that the BM stroma in adult rabbits loses its potential to support B lymphopoiesis. Surprisingly, IL-7, a cytokine critical for B lymphopoiesis, was found to be up-regulated in adult rabbits.…”

“…Surprisingly, IL-7, a cytokine critical for B lymphopoiesis, was found to be up-regulated in adult rabbits. 7 A cDNA-RDA assay to compare cultured stromal cells from adult and young rabbits identified the ECM molecule periostin as a molecule that appeared to be down-regulated in adult BM stromal cells. Periostin is a secreted ECM protein of poorly defined function but has been associated with development of bone, teeth, and heart.…”

“…13 In contrast, B cells are produced in rabbits only during gestation and until ϳ 4 months after birth. 6 Investigating the mechanism by which the observed B lymphopoiesis arrests in adult rabbits, Kalis et al 7 showed that BM stroma from adult rabbits loses its potential to support proliferation and differentiation of HSCs into B-lineage cells. As a result of this finding, we searched for age-related differences in young and adult rabbit BM stromal cells.…”